Thermal energy transfer loops, known as heat pipes, can transfer heat energy efficiently from a heat source to a sink. Circulation of the heat-transfer coolant in the loop advantageously can be driven simply by the heat energy that the loop dissipates. In loops having a coolant phase change, from liquid to vapor at an evaporator that is thermally coupled with the heat source, and from vapor to liquid at a condenser that has capacity to absorb heat energy, the phase changes store and release a portion of the energy being moved. The heat source might be a semiconductor or integrated circuit, for example. The heat sink can be a finned air-heat exchanger for release of heat into the ambient air, another heat exchange fluid or loop, a thermally conductive mass such as a cabinet, etc.
There are design considerations for such heat pipes. The heat pipe typically needs sufficient capacity over a range of operational conditions to hold the temperature of the heat source below a desired maximum temperature. Providing heat transfer capacity may require efficiency and may dictate criteria such as minimum contact areas and/or coolant flow rates. The evaporator and the condenser should have close thermal coupling with the heat source and sink, respectively. To move heat energy, positive temperature differentials must be maintained along the heat transfer path from the source to the sink. The thermal transfer and fluid dynamic characteristics need to convey heat energy over a range of expected temperatures of the source and sink.
Preferably the device is compact and does not interfere unduly with access to structures associated with the heat source and sink. The device should robustly resist damage or deterioration. It should carry minimal expense. These design considerations affect one another. For example, increases in capacity generally also increase size or expense. What is needed is efficient thermal transfer activity and high thermal transfer capacity, in a small and inexpensive device.
Heat energy moves when there is a temperature difference between thermally related bodies, e.g., heat conductive materials in a conductive, convective or radiant relationship. Advantages are achieved if the temperature differences are arranged to produce phase changes in the coolant, i.e., cyclic changes of the coolant that store and release heat energy. The gaseous and liquid phases can diffuse and flow, which is potentially useful to move the coolant in one or both directions between the evaporator and the condenser.
The evaporator is heated by the heat source, causing liquid phase coolant in the evaporator to vaporize. Heat energy from the evaporator is transferred into the coolant by the phase change. The vapor phase coolant dissipates, some flowing to the condenser. The condenser is maintained at a lower temperature than the evaporator. Heat energy is coupled from the coolant into the condenser, dropping the temperature of the vapor coolant. The coolant condenses back to the liquid phase. The liquid phase coolant is conveyed back to the evaporator, for example by gravity flow, by capillary flow through a wick structure or other means. The cycle repeats.
So long as the condenser has an associated means to carry the heat away, the process continuously transfers heat energy away from the heat source. The condenser can dissipate the heat energy into a thermal sink such as the ambient air, using a finned heat exchanger, alone or assisted by forced air or the like.
The heat pipe structure generally involves a substantially closed, typically vacuum-tight envelope coupling the evaporator and condenser, and the coolant or working fluid. It is possible to rely on gravity flow from the condenser to the evaporator if the orientation of the device is assured. Where gravity is not reliable, for example as in portable electronic equipment, a wick between the condenser and the evaporator can provide capillary flow whereby the surface tension of the coolant is sufficient to power the return flow of coolant in the liquid phase. The wick can comprise particulate material adhered to the inside walls of the heat exchanger envelope.
When initially charged, the heat pipe envelope is evacuated and back-filled with a small quantity of working fluid, typically enough liquid coolant to ensure saturation of the wick. The atmosphere inside the heat pipe assumes an equilibrium of liquid and vapor phases. In the absence of a temperature difference between the evaporator and the condenser, the coolant remains more or less stagnant.
Heat energy added at the evaporator generates additional vapor and a slightly higher vapor pressure at the evaporator. Vaporization of the coolant stores a certain amount of thermal energy in the phase change. The vapor diffuses through the envelope to the condenser. At the condenser, the slightly lower temperature causes some of the vapor to condense giving up the stored thermal energy, known as the latent heat energy of vaporization. The condensed fluid flows back to the evaporator, e.g., driven by the capillary forces developed in the wick structure. If the thermal energy output of the heat source should increase, and assuming a constant temperature at the condenser (i.e., if the temperature difference increases), the rates of vaporization and condensation increase, and more heat energy is moved. However, heat energy can be can be moved even at low temperature gradients. The device adapts to dissipate heat as necessary. Its operation is driven only by the heat that it serves to transfer.
It is conventional in cooling integrated circuits for desktop computers, laptops, servers, power regulation devices and the like, to clamp an individual heat sink device to each integrated circuit or similar load that needs to be cooled. This technique contrasts with techniques that would couple the heat energy of several devices to one heat sink, for example typified by audio amplifiers that have several power transistors mounted to a single massive heat sink. If several heat sources are thermally coupled to the same heat sink, the hotter source(s) heat the cooler one(s) and vice versa. The operating temperature of the cooler sources is increased. The temperature gradient, particularly between the hotter source and the ambient, is reduced. Having a lower temperature gradient reduces the rate of thermal transfer to the ambient, i.e., reduces efficiency. It would be advantageous to deal with cooling of sources having different temperatures in a manner that does not simply average their output and instead benefits from the higher temperature gradient made possible by the higher temperature heat source.
Larger and smaller scale heat pipes are applicable to different situations where more or less heat is to be dissipated. The particular coolant can be chosen and its pressure conditions set so as to obtain cyclic phase changes at nominal operational temperatures. There is a challenge in the case of modern integrated circuit devices such as computer desktop and laptop devices, consumer electronics and similar equipment. Such equipment may have multiple digital processors or other large scale integrated circuits, each dissipating energy at a different but substantial rate. It is difficult to provide a heat sink heat pipe optimized for each heat source due to the range of different devices and the different operational and ambient conditions that may arise.
Moreover, providing a separate heat pipe for each of several heat sources generally requires a volume of air space for convection and/or a blower to force air over the individual heat pipes and possibly through the cabinet holding the equipment. What is needed is an efficient way to deal with plural heat sources producing different heat dissipation conditions, while keeping the overall cabinet compact and uncomplicated.